Deviated (non-vertical) wells are often drilled using directional drilling techniques from a gap in the casing of an existing cased wellbore, which gap may comprise a window cut in the casing or a section cut from the casing. A non-rotating drill string having an attached drilling tool is run into the existing wellbore. The drilling tool typically includes a drill bit for drilling the deviated well, a downhole motor for rotating the drill bit without rotating the drill string, an orientating sub for orienting the drill bit and several joints of drill pipe. The drill string connects the drilling tool to the surface of the earth. To commence the drilling of the deviated well, the drill string is run into the existing wellbore until the drill bit is located at the gap in the casing. A survey tool is then run into the drilling tool and into the orientating sub. The drill string is then rotated until the desired bit direction is obtained. The survey tool is then removed. Once oriented, the drill string is lowered until the drill bit contacts the point in the wellbore from which drilling is to commence. The downhole motor is then operated to commence the drilling of the deviated well from the gap in the casing. Once the drilling of the deviated well has progressed for a distance from the gap sufficient to minimize magnetic interference caused by the casing string, typically 200-250 feet, a measure while drilling (MWD) system incorporating accurate but sensitive survey equipment or some other conventional directional drilling tool can be used to continue the drilling of the deviated well.
To drill the well, weight must be applied to the drill bit through the drill string. Typically, the more weight that is applied to the drill bit, the higher the penetration rate of the drill bit. This weight applies an axial load to the drill string, and also results in the generation of reactive torque by the downhole motor as the drill bit is forced against the end of the wellbore. The axial load can cause bending or buckling of the drill string, and the reactive torque can cause the drill string to twist. This bending and twisting of the drill string can in turn create difficulties in maintaining the desired orientation of the drill bit as drilling progresses, and the greater the weight applied to the drill bit, the more serious these difficulties become. The situation is aggravated where the weight applied to the drill bit fluctuates, since this adds a further element of uncertainty to trying to maintain the desired orientation of the drill bit.
To overcome the problems associated with reactive torque when commencing the drilling of a deviated well, and thus drill more accurately during the critical initial stages of directional drilling before the relatively sensitive MWD system can be used, the method of time drilling is often performed. With time drilling, as soon as the drill bit contacts the point from which drilling of the deviated well is to commence and the pump pressure in the downhole motor increases by a predetermined amount, no further weight is applied to the drill bit. The pump pressure is allowed to drop to its original pressure as the drill bit drills out from the commencement point. Once the pump pressure is back to its original pressure, the pressure is increased again by applying a small amount of weight to the drill bit. This cycle is repeated throughout drilling.
It has been found that time drilling is typically relatively costly and time-consuming. The amount of the weight on the bit and the resultant pump pressure must be carefully monitored and maintained within a relatively narrow range in order that orientation of the drill bit can be maintained relatively accurately during drilling. Further, while performing the time drilling method, the orientation of the drill bit is confirmed by the survey tool every approximately 6 to 9 feet drilled. As the survey tool is typically quite sensitive, it cannot be left in the orientating sub during drilling. Thus, drilling must cease every approximately 6 to 9 feet to allow the survey tool to be run back into the orientating sub. If the drill bit direction has not changed, drilling is resumed. If the direction has changed, the drill bit must be reoriented. Thus, again, the entire time drilling process can be very time-consuming.
As well as time drilling, other methods have been developed to assist in maintaining the orientation of the drill bit during drilling. For example, U.S. Pat. No. 4,697,651 issued Oct. 6, 1987 to Dellinger is directed at a method for carrying out directional drilling with a rotary drilling tool by imparting both rotation and a rapid dynamic movement downward to the tool so that the drill bit impacts the end of the wellbore. A drilling stroke is then produced by allowing the tool to move from a contracted position to an extended position and an MWD system is used to monitor the orientation of the drill bit between strokes. In this manner, weight continues to be applied to the drill bit during the drilling stroke. Further, the tool includes spiral bladed stabilizers to effect proper orientation of the drill bit prior to each drilling stroke as the drill tool is raised to move the drill bit out of contact with the end of the wellbore.
As indicated, Dellinger involves a relatively complex series of steps. As well, the weight on the bit may be limited by the extension of the tool from the contracted position to the extended position and the required movement of the drill string between each drilling stroke may cause difficulties with maintaining the orientation of the drill bit. Finally, Dellinger does not appear to contemplate the commencement of drilling of a deviated well from a fully cased existing wellbore.
There is therefore a need in the industry for a drilling tool and method for use in commencing the drilling of a deviated well from an existing cased wellbore which maintains the desired orientation of the drill bit by minimizing the effects of reactive torque and axial loading on the drill string, while allowing a relatively more consistent weight, and a relatively greater weight, to be applied to the drill bit as compared to previously known tools and methods.